This invention relates in general to the field of magnetic bearings and, more particularly, to a system and method for measuring dynamic loads in a magnetic bearing.
Magnetic bearings are used in many industrial applications. Magnetic bearings are used increasingly to support rotating turbomachinery, spindles of machine tools, and other rotating shafts. Magnetic bearings use attraction forces from electromagnets to levitate a machine""s rotor. Because magnetic bearings are inherently unstable, they use relative displacement sensors to assist in active control of the current supplied to the electromagnets to provide stability. In certain applications, such as machine tools for machining, accurate measurement of the dynamic forces developed by the bearing and acting on the rotor is important.
One method for measuring or estimating the dynamic forces developed by a magnetic bearing is to measure control currents supplied to the poles of the bearings. However, this approach results in an inadequate accuracy estimate of between 2% and 10% of the full-scale load capacity of the bearing.
Another method for measuring or estimating the dynamic forces is mounting the bearing on load cells that have conventional electrical strain gauges. However, this approach typically requires an unwanted modification of the machine support structure to accommodate the load cells. Also, a magnetic bearing""s temperature normally increases after start-up. Hence, a conventional electrical strain gauge measures the strain induced by a temperature increase and typically requires calibration at operating temperatures. Piezoelectric load cells are a possible alternative for direct measurement of transient loading, but normally require an even more extensive modification of a machine""s housing for installation. Additionally, piezoelectric load cells normally measure time-varying loads and do not measure the steady load applied by the bearing.
An additional method for measuring the dynamic forces developed by a magnetic bearing is that a Hall flux sensor can be installed at the face of the electromagnets to measure flux and infer forces. However, the gap between the face of the electromagnet and the rotor must be increased to accommodate the flux sensor. This results in a dramatic loss of load capacity.
In accordance with the present invention, a system and method for measuring dynamic loads in a magnetic bearing is disclosed.
In one embodiment of the present invention, a system for measuring dynamic loads in a rotordynamic system having a rotor includes a magnetic bearing disposed around the rotor and having a plurality of magnets and at least one fiber optic strain gage associated with a pole of a respective magnet. The fiber optic strain gage is operable to detect a strain on the respective pole resulting from the dynamic loads. The detected strain is indicative of the dynamic loads.
Some embodiments of the invention provide a number of technical advantages. Embodiments of the invention may include all, some, or none of these advantages. The design of magnetic bearings may be improved by measuring the actual dynamic forces created via fiber optic strain gages. Furthermore, these fiber optic strain gages are able to measure significantly lower levels of strain than conventional electrical strain gages. Furthermore, electromagnetic interference does not affect fiber optic strain gages. Fiber optic strain gages also have exceptional durability over a wide load range and a high tolerance for high temperatures. In addition, fiber optic strain gages can be efficiently installed in magnetic bearings without degrading the load capacity of magnetic bearings.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.